This invention relates to ceramic compositions having a high dielectric constant and more particularly to ceramic compositions suited as a dielectric material for multilayer ceramic capacitors.
Ceramic compositions essentially consisting of BaTiO.sub.3 have been widely used as a ceramic dielectric material of high dielectric constant. However, these BaTiO.sub.3 -system compositions must be sintered at a very high temperature in the range from 1300.degree. C to 1400.degree. C, so that where they are used as a dielectric material of multilayer ceramic capacitors, costly noble metals such as platinum, palladium or the like which can stand such high sintering temperatures should be used as internal electrodes of the capacitors. Therefore, there has risen a demand for ceramic dielectric materials which can be sintered at a temperature as low as below 1000.degree. C for enabling the use of inexpensive metals such as nickel, silver and the like as the internal electrodes.
It is accordingly an object of the present invention to provide ceramic compositions which allow low-temperature sintering and have a high dielectric constant (.epsilon.).
Another object of this invention is to provide ceramic compositions of a low dielectric loss (tan .delta.) and a high specific electric resistivity (.rho.).
Still another object of the present invention is to provide a method of fabricating multilayer ceramic capacitors with a low cost.
The ceramic compositions according to the present invention consist essentially of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 and Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 with a molecular ratio of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 to Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 ranging from about 0.2 to 0.5. The Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 -- Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 ceramic compositions of the invention can be added with at least one compound of SiO.sub.2, NiO, ZnO, Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Mn.sub.1/3 Ta.sub.2/3)O.sub.3 and Pb(Fe.sub.1/2 Sb.sub.1/2)O.sub.3 for improving their dielectric loss (tan .delta.) and specific electric resistivity (.rho.).
Namely, the present invention firstly provides ceramic compositions consisting essentially of 20 to 50 mol % Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 and 80 to 50 mol % Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3. In other words, the ceramic compositions represented by the formula Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3, in which x lies within the range of about 0.2 to 0.5 are provided. These ceramics can be sintered at a temperature not exceeding 1,000.degree. C and have a high dielectric constant (.epsilon.) of 5,000 or more.
The present invention also provides ceramic compositions consisting essentially of Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3 as a basic constituent, in which x lies within the range of about 0.2 to 0.5, and at least one oxide selected from the group consisting of SiO.sub.2, ZnO and NiO as a sub constituent wherein an amount of such sub constituent is in the range of about 0.02 to 1.0 mol % with respect to the whole composition.
The present invention further provides ceramic compositions consisting essentially of Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3 as the basic constituent, in which x ranges from about 0.2 to 0.5, and at least one compound selected from the group consisting of Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Mn.sub.1/3 Ta.sub.2/3)O.sub.3 and Pb(Mn.sub.1/3 Sb.sub.2/3)O.sub.3 as the sub constituent with the molecular ratio of the sub constituents to the whole composition of about 0.05 to 20.0, 0.05 to 10.0 and 0.05 to 5.0 mol%, respectively. In other words, the ceramic compositions consisting essentially of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 x. Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 y .multidot. Pb(Mn.sub.1/3 Me.sub.2/3)O.sub.3 z wherein Me represents one of Nb, Ta and Sb and x + y + z 1.00 are provided, in which x ranges from about 0.1000 to 0.5005, y ranges from about 0.4000 to 0.7995, while z ranges from about 0.0005 to 0.2000 for Nb, from about 0.0005 to 0.1000 for Ta and from about 0.0005 to 0.0500 for Sb.
The ceramic compositions added with the mentioned oxide or compound have a high specific resistivity of 1 .times. 10.sup.9 .OMEGA.-cm or more and a low dielectric loss (tan .delta.) of less than 5% as well as a high dielectric constant of 5,000 or more and can be sintered at a temperature not exceeding 1,000.degree. C.
The present invention also provides a method of fabricating a multiplayer ceramic capacitor which comprises the steps of forming a sheet containing materials necessary to obtain a composition consisting essentially of Pb(Fe.sub.1/3 W.sub.2/3).sub.x .multidot. Pb(Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3 (0.2 .ltoreq..times..ltoreq.0.5) or of [Pb(Fe.sub.1/3 W.sub.2/3)].sub.x [Pb(Fe.sub.1/2 Nb.sub.1/2)].sub.1-x O.sub.3 (0.2 .ltoreq..times..ltoreq.0.5) and at least one of SiO.sub.2, ZnO, NiO, Pb(Mn.sub.1/3 Nb.sub.2/3) O.sub.3, Pb(Mn.sub.1/3 Ta.sub.2/3) O.sub.3 and Pb(Mn.sub.1/3 Sb.sub.2/3) O.sub.3, applying to a surface of said sheet a layer of metal such as Ag paste, nickel, and Ag-Pd paste which is vaporized or corroded at a temperature well over 1,000.degree. C, laminating a plurality of said sheet at least one of which has said metal layer thereon, and sintering the laminated body at a temperature of below 1,000.degree. C and favorably of 850.degree. to 990.degree. C. According to this method of fabricating a laminated ceramic capacitor of the present invention, the sintering temperature to form ceramic dielectric is less than 1,000.degree. C, which makes it possible to use inexpensive metals such as nickel, silver, siver-rich alloy and nickel-rich alloy which cannot stand a high temperature well over 1,000.degree. C as internal electrodes in multiplayer or laminated ceramic capacitors without damage. Accordingly, the cost of fabricating the laminated ceramic capacitors can be highly reduced.